Sparse graphs with an independent or foresty minimum vertex cut

Abstract

A connected graph is called fragile if it contains an independent vertex cut. In 2002 Chen and Yu proved that every connected graph of order $n$ and size at most $2n-4$ is fragile, and in 2013 Le and Pfender characterized the non-fragile graphs of order $n$ and size $2n-3.$ It is natural to consider minimum vertex cuts. We prove two results. (1) Every connected graph of order $n$ with $n\ge 7$ and size at most $\lfloor 3n/2\rfloor$ has an independent minimum vertex cut; (2) every connected graph of order $n$ with $n\ge 7$ and size at most $2n$ has a foresty minimum vertex cut. Both results are best possible.